1. Field of the Invention
This invention relates to the way a personal computer is interfaced to shared local area network (LAN) wiring such as might be installed in an office building. It is particularly useful with portable and laptop computers.
2. Prior Art
Currently, the personal computer user connects his or her computer to the LAN by installing in the computer a LAN adapter card which includes a "pin-and-socket" input/output (I/O) receptacle connector and subsequently connecting between this receptacle connector and the LAN wiring an adapter cable assembly. The adapter cable has on one end, a pin-and-socket plug connector which mates with the receptacle connector on the adapter card, and on the other end a connector which plugs into a wall plate socket which is in turn connected to the LAN wiring installed in the building. The personal computer can be moved from room to room if necessary by simply unplugging the adapter cable, and plugging it in in a different room.
The above scheme becomes more complicated if the user wants to move his or her computer from building to building and the LAN of choice can be operated on two or more different kinds of wiring with different impedance characteristics and wall connectors. For example, the token ring local area network can be operated on shielded twisted pair media or unshielded twisted pair media. Each type of media requires its own drive and receive circuits on the adapter card because each has different impedance and drive characteristics. Users of small notebook or laptop personal computers which are equipped to connect to the token ring LAN are likely to encounter both kinds of wiring as they go to meetings in different buildings, possibly in different geographic areas or even different countries. In the current art, such users are accommodated in one of two ways.
In one scheme, drive circuits for both types of media are kept active and brought out to pins or sockets of the receptacle connector of the adapter card. The user is supplied with two adapter cables. On one the appropriate network wall plug is wired to the appropriate pins or sockets on the plug connector to connect to one type of media; on the other the second type of network wall plug is wired to the appropriate pins or sockets on the plug connector for the other media. The user simply switches adapter cables to switch between different types of LAN media. This scheme works, but the user has to carry two adapter cables. Additionally, the supplier of the LAN adapter card for the personal computer has the added cost of supplying two cables.
In the second scheme, the user is supplied with one adapter cable. The signal conductors and ground conductors for both types of media are wired into the cable and connected to pins or sockets on the plug connector which mate with appropriate pins or sockets on the receptacle connector on the adapter card. Both network wall plug connectors are wired to the other end of the adapter cable. The user can leave the one adapter cable connected to his or her personal computer and plug in whichever network connector is needed. The problem with this system is that both sets of signal wires in the adapter cable cannot remain active at the same time. If they are, the unused and unterminated set of signal wires will give off high levels of electromagnetic emissions, and potentially prevent the system from complying with Federal Communications Commission regulations restricting such emissions. The unterminated wires will also disrupt the transmission line properties of the adapter cable, making the whole system susceptible to incoming electromagnetic interference, return loss from reflected signals on the unterminated lines, jitter and similar problems. In the current single-cable scheme therefore, a relay must be provided on the adapter card to switch between the two different sets of media signal wires in the adapter cable. This relay takes up space in the adapter card and adds expense. It also adds complexity in that the relay must be controlled through software or some type of manual intervention by the user.
What is needed is a way to provide a single adapter cable with good electromagnetic characteristics that can be used with either of two types of media. Such an adapter cable can be made by taking advantage of the inherent switching capability possessed by a symmetrical connector with multiple sets of contacts. U.S. Pat. No. 5,118,312 to Lu, for example, discloses a wall-plate telephone-style connector with two sets of contacts that can be used to switch a single telephone line. Lu, however, provides no way to disconnect the unused telephone line, or to maintain the transmission line characteristics of the line.